Steel sheets for automobiles are intended to have higher strength in consideration of safety of the automobiles and environmental issues. In general, the workability of a steel sheet decreases with an increasing strength thereof. However, a variety of steel sheets having both high strength and satisfactory workability have been developed and become commercially practical. For example, a steel sheet having a composite structure including a ferrite phase in coexistence with one or more low-temperature transformation phases such as martensite and bainite is used as a high-strength steel sheet excellent in workability. The steel sheet having such a composite structure is designed to improve both the strength and workability by dispersing a hard low-temperature transformation phase in a soft ferrite matrix. Such steel sheets having a composite structure, however, suffer from work fracture starting from inclusions.
Under these circumstances, there have been proposed techniques for improving the workability by controlling inclusions. Typically, Japanese Patent No. 3845554 describes that a cold-rolled steel sheet excellent in bending workability is obtained by controlling the number of inclusions to 25 or less per square millimeter (mm2), which inclusions have diameters in terms of corresponding circles of 5 μm or more. Japanese Unexamined Patent Application Publication (JP-A) No. 2005-272888 describes that a highly ductile cold-rolled steel sheet is obtained by controlling the number of oxide inclusions to 35 or less per square centimeter (cm2) in a silicon-deoxidized steel, which oxide inclusions have minor axes of 5 μm or more. This document also mentions that inclusions are finely divided by controlling the composition of inclusions to one which is liable to expand and break.
However, even when individual inclusions are finely divided and dispersed at a low number density as in the techniques disclosed in the two documents, fracture or cracking starting from inclusions may occur in some distributions of the inclusions. Further investigations are needed so as to reliably increase the workability, especially bending workability required of steel sheets for automobiles. The technique disclosed in Japanese Patent No. 3845554 requires the steel to be a low-sulfur steel, and this leads to increased cost. Japanese Unexamined Patent Application Publication (JP-A) No. 2005-272888 does not refer to the bending workability required of steel sheets for automobiles, among such workabilities.
Independently, Japanese Patent No. 3421943 describes that can-making (plate working) failure of a cold-rolled steel sheet for cans is reduced by controlling the abundance of dot-sequential inclusions to 6003 per square meter (m2) to 2×104 per square meter, in which the dot-sequential inclusions are observed in an arbitrary cross section in parallel with a rolling plane of the steel sheet. The dot-sequential inclusions herein are a group of three or more oxide inclusions that are arranged linearly at intervals of less than 200 m in parallel with the rolling direction. The steel disclosed in the document, however, is adopted only to cans and needs drawing workability. However, the document does not consider the bending workability needed when used as the steel sheet for automobiles.
As is mentioned above, known high-strength steel sheets with fewer defects caused by inclusions are obtained mainly by strictly controlling the sizes, numbers, and/or amounts of individual inclusions. However, the known steel sheets, when subjected to bending, may suffer from fracture generated sporadically even under remarkably mild working conditions. This lowers the productivity and causes increased cost due typically to inspection of products.